ON THE PRESENCE OF GENIAL TUBERCLES ON THE MANDIBLE OF MAN, AND THEIR SUGGESTED ASSOCIATION WITH THE FACULTY OF SPEECH.' By ARTHUR THOMSON, Professor of Human Anatomy, University of Oxford. AT a time when so much interest is centred around the discovery of early human remains, it may not be amiss to draw attention to certain considera- tions in relation to the morphology of man's mandible which may have a bearing on the elucidation of some of the problems which have arisen in connexion with the inferences to be deduced from a study of the osseous fragments. Confining my attention, meanwhile, to the study of the lower jaw, we have now a considerable number of " fossil " specimens that display characters which may be regarded as unusual in living races. Among these we may mention the reduction in size of the mental protuberance and tubercles as displayed in the famous Heidelberg jaw, and also ex- emplified in the mandibles from Spy, that of Naulette, the Moulin Quignon jaws, La Chapelle aux Sainte and the Moustier remains, all of which exhibit an ape-like appearance in the slope of the anterior symphysial surface. As long ago as 1867 Broca2 drew attention to this condition, and clearly proved that instances were to be met in the mandibles of recent races, in which these characters were as pronounced as in the case of the so-called fossil types. He illustrated this in his memoir by a figure of the mandible of a New Caledonian, and I am fortunate enough to be able to confirm this observation by an equally well-pronounced specimen from the same locality, at present deposited in the Williamson collection of skulls (No. 300) of the Royal Army Medical Corps at Millbank, London. This New Caledonian mandible, of which I give a figure (fig. 1), exhibits the same roundness and absence of chin as that represented by Broca, and sets at rest the assumption that there is anything exceptional in the occurrence of a like or similar condition in the jaws of fossil man. That the occur- rence of this type of mandible is not very unusual is common knowledge, though it is rare to meet with it in such characteristic form as that 1 The substance of this paper was communicated to a meeting of the Anatomical Section of the International Congress of Medicine held at the Royal College of Surgeons, London, on 11th August 1913, but its publication has for various causes been delayed. 2 Mimoires d'anthropologie, tome ii. p. 146. 44 Professor Arthur Thomson exhibited in the figure. It is not with this feature, however, that I am immediately concerned. Equally interesting is the observation that in some of the fossil mandibles there is a pit or fossa in place of the raised tubercular area to which the genial muscles are attached in man. This has led to many surmises as to its significance; for, whilst most have agreed that the occurrence of this anomaly points to a similarity between it and the appearance exhibited in the anthropoid apes, not a few have assumed that the presence of such fossis is indicative of a feeble development of the muscles attached thereto, and in particular of the genio-glossus; and in consequence have suggested, nay, well-nigh decided, that the occurrence I IFg...,.".. MN FIG. -1.-andible of New Caledonian. of this feature was an indication that man had not yet reached that stage in his evolution when he had acquired the faculty of articulate speech. So far-reaching an assumption demands our most serious con- sideration before we can admit its acceptance. The appearance of the lingual surface of the symphysial part of the mandible in the anthropoid apes is so well known that little time need be spent on its description. In place of a tubercular area for the attachment of the genial muscles, such .as we meet with in man, the corresponding site reveals a well-marked fossa or hollow, which may conveniently be called the genial fossa; the depth of this fossa is still further emphasised by the presence of a transverse ledge or shelf which unites the lower borders of the body of the mandible on either side, behind and below the symphysis. This, oftentimes known as the simian ledge, may be better termed the digastric plate, for reasons that will hereafter be explained. The presence of this ledge in the apes materially shortens the arch of the jaw inferiorly, On the Presence of Genial Tubercles on the Mandible of Man 45 and so limits to a corresponding extent the area occupied by the tongue and its associated muscles, whilst at the same time it deepens the genial fossa, and so renders more striking the contrast between the appearance displayed in this region in the ape and that exhibited by man in the corresponding situation. On a more careful examination of this region in the anthropoids, it will be seen that there are individual differences exhibited in the appearance of the parts in members of the same species. In the gorillas I examined, I met with a more pronounced bulge on the upper part of the posterior surface of the symphyses in some instances than in others, in consequence of which the thickness of the mandible was much increased, measuring in one specimen as much as 27 mm. with a thickness of 20 mm. through the bottom of the genial fossa, whilst in another equally mature specimen of the same sex the corresponding measurements were only 195 mm. and 12 mm. respectively. In consequence of this difference in the symphysial thickness, there was exposed in the latter example a larger area of the upper surface of what I may term the digastric plate, and thereby a greater appearance of depth was imparted to the genial fossa. Usually the floor of the fossa was pierced by a pair of vascular foramina, one on either side of the middle line. Sometimes these foramina were not of equal size and not necessarily on the same horizontal line. A median crest, but faintly marked, appeared above, between these two foramina, and, running downwards, passed over the upper surface of the digastric plate to end inferiorly on its posterior margin in an irregular tubercle, sometimes double. On either side of this median line the bone was occasionally slightly roughened, suggesting the attachment of tendinous fibres to its periosteal surface. There was nothing to indicate the precise attachment of the genial muscles, and it was impossible to differentiate between the attachment of -the genio-glossus and the genio-hyoid. They might, from such indications as appeared on the bone, have been a combined mass, the position of the median line merely serving to separate the right and left fleshy masses from each other. In two of the Oxford gorillas the under surface of the digastric plate exhibited different features; in one (a a) it was smoothly confluent with the rounded surface of the sloping symnphysis externally, whilst in the other (b b, 2052) it was recessed within the converging margins of the lower borders of the jaw, which swept anterior to it to meet in front at a point which may be regarded as the inferior extremity of the symphysis, assuming that to be independent of the digastric plate. These appearances suggest the necessity of a further inquiry as to the 46 Professor Arthur Thomson precise attachment of the various muscles connected with the bone in this region, for possibly that which I have designated as the digastric plate may in all probability be a more extensive surface for the attachment of the genio-hyoid. In the Oxford chimpanzee (2049 b) the maximum symphysial thickness was 13 5 mm.; the minimum thickness of the bone between the bottom of the genial fossa and the external surface of the symphysis was 5-5 mm. From the fossa, which was funnel-shaped, there passed a deep pit into which there opened three foramina of unequal size. The upper surface of the digastric plate, which in this instance was short, was unmarked by a ridge, but its posterior edge was emphasized by the presence of a short and stunted spine situated in the middle line. The under surface of the digastric plate was everywhere confluent with the rounded surface of the mental part of the bone. On either side of the middle line, 13 mm. below the alveolar border, there was a small vascular foramen with a groove leading up to it. In the orang the arrangement closely resembles that of the gorilla. In one specimen (d) belonging to the Oxford collection the maximum thick- ness of the symphysis was 18-5 mm.; the minimum thickness in corre- spondence with the bottom of the genial fossa was 11 mm. In the other example from the department of human anatomy the maximum thickness was 19-5 mm.; the minimum at the fossa was 10 mm. In both, the fossa was pierced by three vascular foramina, one large and two smaller; the latter were placed one above the other, and lay in one specimen (d) to the left of the middle line, in the other (H.A.D.) to the right. The larger foramen in both examples pierced the bone on the opposite side to that on which the smaller canals were situated. In both the digastric plate was well developed, though in one its upper surface was more extensive than the other. In each case there were indications of a faint miesial line which blended posteriorly with a small tubercle on the posterior edge and' slightly on the under surface of the plate.